Superkuh2 demonstrates how to decode FLEX pager radio traffic. He uses gr-pager and zarya’s implementation of it on the Linux platform with the RTL-SDR dongle.
This script runs at a 250 KS/s sample rate and decodes one 12.5 KHz channel only. Internally it uses gnuradio’s optfir to generate low pass taps stopping at 12.5 KHz to use witih a frequency xlating FIR filter. It then passes what’s left to gr-pager’s flex_demod.
Here I show such an activity with the osmocom gr-fosphor visualization tacked on to patchvonbraun’s gnuradio multimode receiver for context.
For more detail on this process, visit SuperKuh’s website.